1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, or a facsimile machine, and more particularly to rotational phase control of an image bearing member, such as a photosensitive drum, which is performed by a color image forming apparatus.
2. Description of the Related Art
In recent years, image forming apparatuses (copying machines, printers, facsimile machines, etc.) using an electrophotographic process have been experiencing a shift from a monochrome apparatus (monochrome image forming apparatus) to a color apparatus (color image forming apparatus).
As color image forming apparatuses, there are generally known a so-called single drum type and a so-called tandem type. In a tandem-type color image forming apparatus, toner images in respective colors are formed on respective associated photosensitive drums arranged in a predetermined direction as image bearing members. The toner images in the respective colors are sequentially transferred onto an intermediate transfer member (intermediate transfer belt) as one of image bearing members, whereby a color toner image is formed. Then, the color toner image on the intermediate transfer belt is transferred onto a recording medium (e.g. a recording sheet), whereby a color image is formed. In the tandem-type color image forming apparatus, each photosensitive drum and the intermediate transfer belt both function as image bearing members.
The tandem type is inferior to the single drum type in size and cost reduction. However, the tandem type is capable of performing an image forming operation on a color-by-color basis, and is therefore suited to speed up the image forming operation. For this reason, among the color image forming apparatuses, from the viewpoint of the speed of image formation, the tandem type which can provide substantially the same image forming speed as the monochrome apparatus is drawing more attention.
By the way, since the tandem-type color image forming apparatus has a configuration that toner images of various colors are transferred from the respective photosensitive drums onto the intermediate transfer belt in superimposed relation, there sometimes occurs deviation in position between the color toner images. As a consequence, in the tandem-type color image forming apparatus, color shift occurs, which causes degradation of image quality. In particular, periodic color shift is liable to occur due to run-out of the shaft of each photosensitive drum as a rotary member, irregular rotation of the photosensitive drum, or irregularity of the rotational speed of the intermediate transfer belt.
In order to prevent occurrence of the above-mentioned periodic color shift, it is a practice to control the rotational phases of the respective color photosensitive drums individually.
However, since one of the photosensitive drums and the intermediate transfer belt are in contact with each other during image forming operation, there is a fear that the execution of rotational phase control in the contact state can cause scratching of the photosensitive drum(s) and the intermediate transfer belt due to friction therebetween. If the photosensitive drum(s) and the intermediate transfer belt are scratched, image quality is inevitably degraded.
To prevent such scratching due to friction, there has been proposed a tandem-type image forming apparatus in which when performing adjustment of the rotational phases of respective photosensitive drums, at least photosensitive drums subjected to the adjustment and an intermediate transfer belt are made separate from each other (see Japanese Patent Laid-Open Publication No. 2007-47311).
Further, there has been proposed another tandem-type color image forming apparatus in which to prevent occurrence of scratching even in a state where each of photosensitive drums and an intermediate transfer belt are in contact with each other, rotational phase difference (hereinafter also simply referred to as “phase difference”) between photosensitive drums is corrected by causing the photosensitive drums to rotate at respective rotational speeds made different to such an extent that the service life of each of the photosensitive drums and an intermediate transfer belt is not considerably reduced by friction due to differences in rotational speed between the photosensitive drums and the intermediate transfer belt (see Japanese Patent Laid-Open Publication No. 2001-134039).
In the method disclosed in Japanese Patent Laid-Open Publication No. 2007-47311, by performing rotational phase control of the photosensitive drums in a state where the photosensitive drums and the intermediate transfer belt are made separate from each other, it is possible to prevent scratching due to abrasion caused by difference in rotational speed between the photosensitive drums and the intermediate transfer belt. However, it takes time to make the intermediate transfer belt separate from the photosensitive drums and bring the intermediate transfer belt into contact with the photosensitive drums, and as a consequence, it results in an inconvenience that it takes long before an image is transferred onto a recording medium and is output.
On the other hand, in the method disclosed in Japanese Patent Laid-Open Publication No. 2001-134039, it is required to correct phase difference without increasing the rotational speed of each of the photosensitive drums, so as to prevent scratching due to friction. For this reason, when phase differences between the photosensitive drums are large, it takes much time to control the rotational phases of the respective photosensitive drums.
As can be understood from the above, both of the above-described methods have a difficulty in achieving both prevention of scratching (i.e. degradation of image quality) and reduction of time required for rotational phase control at the same time.